The invention relates to an ion-conductive, thermoplastic composition containing a partially acetalated polyvinyl alcohol, at least one support electrolyte and at least one plasticizer, and to the use, in electrochromic composite glazing, of thin foils produced therefrom.
Composite glazing, whose transparency and/or color can be modified by applying an electric voltage, is referred to in the literature as electrochromic glazing. Typically, electrochromic glazing according to FIG. 1 is built up as follows: glass pane (a)—transparent electrically conductive layer (b)—electrochromic layer (c)—solid electrolyte (d)—redoxable ion storage layer or electrochromic layer (e) complementary to (c)—transparent electrically conductive layer f)—glass pane (g).
Layers c) and e) are separated from each other by a solid electrolyte (d). When applying a voltage to the electrodes b) and f), layers c) and e) are oxidized and/or reduced electrochemically, as a result of which their color and translucence is modified in the case where the layers c) and/or e) are electrochromic layers. The oxidation and reduction of layers c) and e) are accompanied by an exchange of ions with the solid electrolyte d). Consequently, this must have a sufficiently high ion concentration. Moreover, a correspondingly high ion conductivity of the solid electrolyte is required for a rapid switching operation.
A solid electrolyte for electrochromic glazing must also exhibit chemical and electrochemical stability as well as optical transparency, apart from a sufficiently high ion conductivity. Many different materials have already been proposed for the production of solid electrolytes for electrochromic glazing.
In European EP 1 056 097, homopolymers or copolymers of acrylic, methacrylic or styrene compounds in combination with plasticizers and supporting electrolytes and with particles of an inorganic or polymeric filler are disclosed for the production of a solid electrolyte.
U.S. Pat. No. 5,244,557, and EP 392 839, EP 461 685 and EP 499 115 describe solid electrolytes based on polyethylene oxide.
In commercial composite glazing without electrochromic properties, intermediate foils of polyvinyl butyral (PVB), i.e. a partially acetalated polyvinyl alcohol, are frequently used. Polyvinyl butyral foils have the advantage of exhibiting a high transparency and provide the glass laminates made therefrom with a good mechanical strength.
The use of polyvinyl butyral foils in electrochromic composite glazing is consequently also known and disclosed, e.g. in EP 1 227 362 and EP 0 657 897. The ion-conductive polyvinyl butyral foils proposed in these applications are produced from conventional PVB resin, plasticizers and support electrolytes as well as, if necessary, further additives. In order to guarantee a sufficiently high ion conductivity, however, a higher plasticizer content is required in this case than in conventional, non-ion-conductive PVB foils. A higher plasticizer content negatively influences the mechanical properties of the foil. Polyvinyl butyral foils in electrochromic glazing consequently possess either an insufficiently high ion conductivity with a good mechanical stability or—in the case of an increased plasticizer content—an improved ion conductivity with a reduced mechanical stability.